Sensitivity Improvement of Laser Microphone Using Self-coupling Effect of Semiconductor Laser

Characterization procedures of different types of semiconductor lasers in a typical transmission line are investigated, with particular attention paid to unavoidable low-level optical feedback. The asymmetric response between front and rear facets is compared with the symmetric response of the solitary device.

The performance of coherent pulse detection (CPD) and multichannel coherent detection (MCCD) based on a single dual-balanced homodyne receiver was experimentally demonstrated using a gratingcoupled hybridly mode-locked semiconductor laser. Compared with direct detection, a high coherent gain of over 10 dB, as well as an SNR improvement of over 5 dB, was obtained in both detection schemes. Our experimental results have confirmed that the coherent detection processes in CPD and MCCD are nearly the same based on a square-root LO power dependence. Nevertheless, the MCCD scheme has shown an advantage in a path-length error over the CPD scheme, revealing 2~3 dB improvement in sensitivities.

To distribute microwaves over fibers, optical single-sideband (SSB) modulation signals are preferred to optical double-sideband (DSB) modulation signals. This study investigates an optically injected semiconductor laser at period-one nonlinear dynamics for optical DSB-to-SSB conversion. For the operating microwave frequencies up to 40 GHz investigated in this study, the proposed system regenerates or even enhances the microwave features of an optical DSB input while converting its optical feature into SSB with an intensity difference of at least 20 dB. The bit-error ratio at 622 Mb/s is down to 10(-9) with a sensitivity improvement of up to 3 dB. The proposed system can be self-adapted to certain changes in the operating microwave frequency and can operate stably under certain fluctuations in the input optical power and frequency.

In a conventional microphone, the diaphragm limits the detectable sound frequency range. To overcome this limitation, a laser microphone was developed using the self-coupling effect of a semiconductor laser. In this paper, the supersonic band characteristics of the laser microphone were investigated using a super tweeter. From the experiment results, it was observed that the optimum reflectivity of a reflector is almost 25% when the laser power is 25 mW. Moreover, a flat frequency response is achieved between 40 Hz and 150 kHz when the long diameter of the laser beam is 1.0 mm, and the upper limit of the detectable frequency relates to the long diameter of the laser beam.

A superregenerative receiver scheme developed for radio communications is translated to the scope of optical communications. Starting from application of the quasi-deterministic theory to the single-mode Langevin rate equations, closed expressions for basic parameters and features, such as bit-error rate and frequency response, are obtained. A comparison of superregenerative receivers with well-established optical receivers shows that improvements in direct-detection receiver sensitivities of more than 10 dB can be obtained; this places superregenerative receiver sensitivities closer to the shot limit. Moreover, the intrinsic frequency selectivity of the superregenerative scheme makes it especially suitable for wavelength-division multiplexed systems. Finally, appropriate devices for implementation of this receiver are suggested.

The slope sensitivity of an active ring laser gyro can be improved by decreasing the group index and the wavelength without having to increase the size of the ring. The nonlinear interaction between a strong driving wave and a probe wave (which is in the spectral vicinity of the driving wave) results in modification of group index of the probe wave. The reduced group index can be used to improve slope sensitivity (scale factor) of a ring laser gyro. By using an InGaN-based multiple quantum well (MQW) semiconductor laser the sensitivity can be further improved, because the lasing wavelength is shorter and the nonlinear interactions between the modes in this material system are also stronger when compared with GaAs-, InGaAs-, and InGaAsP-based laser systems. The finite element method is used to model a realistic InGaN/GaN quantum well waveguide structure. The induced dispersion is calculated and variation of the group index is obtained by the numerical modeling. The spectral position of the critically anomalous dispersion point is shown in wide range of driving wave intensity.

The present generation of optical transmission systems is based on the 1.3um wavelength range where conventional singlemode fibers have low attenuation and dispersion close to zero. The minimum attenuation of these fibers, however, lies close to 1.55um. At this wavelength transmission systems using semiconductor laser sources with launch powers of 1mW and conventional PIN-FET receivers could potentially operate with inter repeater spacings in excess of 100km. Improvements in transmitter power or receiver sensitivity would increase this spacing still further. The introduction of systems operating at 1.55um, however, has been inhibited by the dispersive nature of conventional silica fiber at this wavelength. Typical dispersion is 18ps/nm Km. The characteristics of the source therefore have an important effect on the overall system performance limiting the distance/bandwidth product that can be achieved. This paper describes the development of a range of 1.55um laser sources. It analyses their characteristics as components and shows their impact on system performance parameters.© (1987) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

When a part of the scattering light from the target is entering to the laser cavity, the terminal voltage of semiconductor laser diode slightly fluctuates. This voltage fluctuation has the information related to a distance to the target. However, the terminal voltage fluctuation has many noises. Discrimination of distance signal with many noises is almost impossible. Therefore, a statistical signal processing which utilizes the all information in terminal voltage fluctuation is newly proposed. A system is constructed using Field-Programmable Gate Array (FPGA). This system can measure the distance of 21cm to 45cm with the average error of about 2 %.